1. Field of the Invention
The invention relates to methods for protecting typewriters or similar office machines in operation against the consequences of electrostatic discharges and to circuit arrangements for executing the methods.
2. State of the Art
Modern typewriters or similar office machines usually contain a so-called programmable control unit. This involves at least one microprocessor connected via data lines and address lines to a ROM containing the control program and to a RAM for storing variable data. Through driver circuits the programmable control unit controls electrical drive elements which in turn act upon mechanical devices such as type carriers, type carrier carriages, paper carriers, etc. Since access to the inside of the machine must be provided to change ribbon cassettes, type carriers, etc., the machine housings, usually made of plastic, have, in most cases, a flip-up cover.
In connection with a flip-up cover it is known in typewriters or similar office machines to provide a cover switch. This cover switch causes the programmable control unit to "stop" a control cycle in progress, i.e. directly before possible manipulations, upon opening the cover and to maintain this status for the period of time in which the cover is open. This "stopping" is accomplished, for instance, in that the cover switch, upon opening the cover, causes, by a change of its switch position, the microprocessor indirectly to branch out of the control cycle in progress into a waiting loop. The nature of the waiting loop is such that the microprocessor scans the cover switch cyclically as to a status change, again indirectly. When the cover is closed, the microprocessor detects this status change and branches back into the original control cycle, possibly after executing a so-called turn-on routine. It is mentioned here for the sake of completeness that movable parts (type carrier, type carrier carriage, etc.), possibly moved out of their original position are restored by the turn-on routine.
The reason for the above described procedure is, on the one hand, to prevent reliably unintentionally triggered motions of machine parts made accessible by opening the cover to preclude any danger of injury, and on the other hand, to make it possible to continue the control cycle at the point at which it was "stopped" after closing the cover.
Now, it has turned out that the manipulations involved in changing ribbon as above described cause an electromagnetic interference radiation to originate due to the discharge of electrostatic charges from the person performing the manipulations through jump sparking to conducting grounded machine parts. This electromagnetic interference radiation often changes the status in which the control cycle was stopped by changing data, addresses or flags stored in registers and buffers. The result thereof is that printing results obtained upon the continuation of the control cycle do not coincide with the desired printing result.
In connection with influencing the program flow in computers through electromagnetic waves which may stem, for example, from static discharges through jump sparking, it is known to shield the computers, or at least parts thereof, so as to be insensitive to electromagnetic waves to a large extent. It is also known to take measures to prevent the occurrence of such static discharges in the immediate vicinity of the endangered parts (processors). This can be accomplished, for instance, by insulating all grounded metal parts located in the danger zone.
While the above described measures solve the problem, they are associated with considerable costs and, moreover, cannot be used in all cases.
In connection with computers it is known, furthermore, to branch the program in progress, upon the occurrence of certain events such as failure of the voltage supply, exceeding a certain temperature, etc., into a so-called rescue routine which stores in an external memory all data, addresses and flags required to process the program further so that the program can be continued at the point of interruption after the elimination of the fault.